Patent Application
20110080931
1. Field of the Invention
The present invention relates to laser devices; more precisely, it relates to laser devices which use a laser resonator that has a planar geometry in order to be adapted for a laser vertical emission, i.e. orthogonal to the plane of the same laser resonator.
In particular, but not exclusively, the invention is adapted to be associated to Quantum-Cascade Laser type emitters, which normally have working frequencies in the range of Terahertz (THz QCL).
2. Description of Related Art
In addition to traditional Fabry-Perot (FP) laser devices, which utilise two parallel reflectors that force the radiations to be emitted according to a longitudinal direction, and are associated with waveguides to limit otherwise directed radiations, laser devices are known which use a different emission geometry; among the latter, there are laser devices that utilise circular geometry resonators.
With respect to a traditional FP laser device, in a circular laser resonator the terminal mirrors of the resonator are missing, and the radiations are blocked along a circular path. More precisely, by using as a waveguide a high refraction index material, the obtained photons that are close to the circumference are reflected at a predetermined angle that allows a total reflection, such that the photons remain within the resonator. The modes that resulting from this structure are called “whispering gallery”, due to the analogy with the well-known acoustic phenomenon that takes place under particular architectural vaults. Thanks to this structure, dispersions are reduced to a minimum value, and depend mainly on diffusion phenomena, which are caused for example by circumference imperfections.
However, the practical utility of circular geometry resonators is compromised by a low power output that, in addition, is spread along the laser resonator plane.
Concerning a possible integration with terahertz laser devices (THz QCLs), with the existing laser systems the waveguides operate according to propagation modes whose transversal sections are quite smaller than the wavelength that the emitted radiation would have if such radiation were emitted in open space. Since this radiation is emitted through the cut edge of a waveguide, a strongly diverging output beam of radiations can be obtained, which is therefore of scarce practical use.
On the other hand, the use of vertically emitting recesses (Vertical Cavity Surface Emitting Laser—VCSEL) is known, which makes it possible to obtain radiation beam profiles that are very useful both in the visible range and in the near infrared range. However, the VCSEL cannot be used with devices that are based on the quantum-cascade principle, owing to the rules of selection of the electronic transitions that prevent the emission of radiations in vertical direction.
A circular geometry resonator is known which has a disc or ring like shape, and uses a waveguide structure that furthermore acts like a vertical barrier, i.e., a barrier which is perpendicular to the radiation guide plane. Nevertheless, a disc resonator is described in EP1544967A1, which can be used also for laser emission, and which has a lattice that allows vertically bundling the outlet radiations. The disc can resonate in a whispering gallery mode if a substantially circular recess is created. The lattice can be used in order to select the azimuth mode, i.e. the circumferential mode, for the laser action, according to the mechanisms of distributed feedback (DFB) lasers. In particular, the lattice is made on the disc plane and, for example, it can be structured on the boundary of the disc plane by means of radial cuts.
However, a resonator as described in EP1544967A1 cannot ensure an actual whispering gallery operation mode, and cannot assure that the maximum of the emission is performed in vertical direction.
In fact, the advantages of such a resonator are that vertically emitting planar laser devices can be made. In particular, only if the lattice is a second order one, i.e. only if the lattice pitch corresponds to the radiation wave length, or if a higher order lattice pitch that is multiple of the second order, i.e. if the lattice pitch is a multiple of the radiation wave length, the maximum radiation intensity is obtained in a vertical direction.
However, in a circular symmetry laser device, once the wavelength has been fixed, the most preferred laser emitting mode may also provide a vector of propagation which has a radial component, such that the lattice, even if it has a pitch that corresponds to the radiation length, behaves actually as a first order lattice, and the maximum of the radiation intensity that is emitted by the laser resonator is remarkably diverging, and is of scarce practical use.
In EP0533390, JP06152047, JP5090669 a resonator is disclosed for obtaining circular laser device having perimetral grooves. The number of the grooves is a prime number or an odd number multiple of a prime number respectively 5, 25 and 65.
In devices of this type the problem arises that in order to increase the radiation extraction efficiency the ratio between the surface of the grooves and the surface of uncut zones should be as large as possible, for example 50%. Such large grooves, however, particularly with long wavelength devices, such as THz QC laser devices, reduce the current injection along the perimeter.
Further resonators are known of linear type, such as ridges, buried heterostructure waveguides, etc., in which laser radiation is also extracted in the vertical direction by means of second-order periodic grating which also provides the feedback, so called second-order distributed feedback lasers.
Such linear second-order DFB resonators have certain drawback, particularly when they are applied to THz QCLs operating in metallic waveguides. In particular, such grating resonators provide two lasing modes, i.e. symmetric and anti-symmetric with respect to the grating. However, only one, i.e. typically the symmetric one, is able to efficiently radiate in the vertical direction, due to an interference effect. The other, typically the asymmetric one, produces a much lower intensity and a dual-lobe beam profile and is, therefore, much less appealing for industrial applications. The latter mode, however, since it presents lower losses, is then most often the only one capable of reaching the lasing condition.
It is a feature of the present invention to provide a laser resonator that allows a laser emission in the terahertz frequency range, and, in particular, a laser resonator that has a quantum-cascade active region.
It is also a feature of the present invention to provide a laser resonator that allows a whispering gallery type operation mode in which the maximum radiation is obtained in a vertical direction.
It is also a feature of the present invention to provide a laser resonator that allows emitting a regular profile, low divergence and high efficiency radiation.
It is a further feature of the present invention to provide a resonator that makes it possible to control the efficiency of the vertical emission from the grating mode radiating vertically, its optical losses, as well as the amount of feedback.
It is a further feature of the present invention to provide a resonator that makes it possible to maximise the output power in the vertical direction, at the same time ensuring that only the symmetric mode reaches the laser threshold, producing regular, single-lobe beam profiles.
These and other objects are achieved by a planar resonator which is adapted to be associated to a laser, said resonator comprising an planar active region, a first and a second wave-guiding layer that contains said active region; wherein said resonator has a shape that is defined by a perimeter, along said perimeter said first wave-guiding layer having a plurality of cuts that form a lattice, wherein said cuts are made by at least two adjacent slits and a central zone between the slits in which metal is present.
This way, a cut is defined by a central zone in which metal is present between two slits, or two zones in which metal is present between three slits, etc., achieving the same extraction efficiency of a cut in which the metal is totally removed. This way, the electric pumping efficiency of the cut zone is increased, which is particularly useful in the case of very high wavelengths.
In a preferred embodiment, the perimeter can be circular, and the cuts are radial cuts, wherein the number of said cuts can be a prime number or an odd number that is a multiple of a prime number, said prime number greater than or equal to five.
This way, the formation of resonance modes that extend out of the cut zone or which have a wave vector with a zero component in the radial direction is avoided. Therefore, a real whispering gallery type operating mode is obtained, in which the maximum of the radiation takes place in a vertical direction, i.e. orthogonally to the plane of the laser resonator.
In particular, when the number of radial cuts is a prime number, the laser action surprisingly does not evolve according to one of the resonator modes by the which lattice behaves as if it were an even order lattice; instead, the lattice behaves as if it were a second or higher order lattice.
More precisely, let λ be the wavelength of the radiations as emitted by the laser, Λ be the lattice pitch, i.e., the distance between two cuts, and K=2π/λ, be the wave vector of the radiations, the lattice behaves as a second order lattice, if the component of K in the circumferential direction of the lattice is 2π/Λ, whereas the lattice behaves as a first order lattice if the component of K in the circumferential direction of the lattice is π/Λ. This is possible, even if a same λ value is kept, which has a radial component that is different from zero. This radial component causes, however, a preferential emission direction that is no longer vertical.
Instead, if the number of the cuts along the laser resonator boundary is a prime number, the lattice does not allow the circumferential component to be π/Λ; therefore the lattice behaves like a second order lattice and ensures a vertical laser emission.
A similar situation occurs for frequencies set in the range of terahertz, in the presence of a number of cuts that is not a prime number, but is an odd number that is a multiple of a prime number, where the prime number is greater than or equal to five. In this case, in fact, since the resonator is circular, even if it is theoretically possible that the resonator “lasers” in zones other than the lattice, i.e., towards the centre of the disc, at a distance that is a multiple of the wavelength, such areas cannot “laser” due to the small diameter of the disc.
Therefore, if a laser disc is integrated to a lattice in such a way that the lattice is directly manufactured on the laser disc circular boundary, and that the number of the cuts is chosen according to the above, two important results are obtained:
The lattice can be made by means of photolithography and etching techniques at any vertical position of the radiation guide, in the active region, above or below the active region, or within the coating. In alternative, it can be obtained by laying a plurality of layers on the radiation guide, such layers having the shape of a lattice. In case of a ring laser, or of a disc laser, the lattice can be made both on the circumference, and along the radius of the disc.
A particular embodiment of the invention is a terahertz quantum-cascade laser (THz QCL). This is obtained by interposing a semiconductor active region between two metal waveguides, and by associating it to a lattice that has been formed by radial cuts at the boundary of the disc.
This advantageous wave guide embodiment, which comprises a double metal layer and a lattice, is explained below. An important feature of the manufacture of such laser devices is the use of surface plasmons (SP) to make the resonator, i.e. the use of electromagnetic signals that exist at the interface between two materials that have two different dielectric constants values, one of which is positive, as in case of a semiconductor, and the other is negative, the other material being normally a metal. These optical ways travel along the interface between the two materials and decay exponentially and perpendicularly to the interface. Their intrinsic transversal-magnetic TM polarization obeys to the selection rules of “intersubband” transitions. Furthermore, since the mode achieves peak values at the interface, it can be easily changed to create a patterning on the metal layer, i.e., on the lattice, which therefore would change the spectral features of the resonator.
Consequently, by introducing a periodic corrugation on an SP wave guide a quantum-cascade laser is obtained with a distributed feedback (DFB) in the terahertz range frequencies with steady emissions in a single mode. This conditioning step of the resonator mode, along with a typical wavelength that is two orders of magnitude higher than the wavelength of the optical laser, makes the quantum-cascade laser adapted to develop new concepts of resonators. Furthermore, the wavelength of the radiations in the field of THz and the strong influence of the perimeter of the resonator, which is typical of the wave guide metal, allows an easy production of the lattice. Then, owing to the invention, it is possible to obtain a semiconductor quantum-cascade laser in the field of THz obtaining for these frequencies a vertical laser emission.
Advantageously, said laser resonator has a semiconductor active region that is arranged between two doped semiconductor layers, apart from a central zone, where the doped layer is missing. This way, any emission is prevented from leaving the central zone of the laser resonator, thus forcing further the device to emit radiations in a whispering gallery mode, only in the lattice region. This is particularly useful if the coupling coefficient of the lattice, indicated as κ, is particularly small, in particular κ<1/L, where L is the length of the circumference.
Advantageously, said laser resonator has said first and second wave-guiding layers that are made of a metal, preferably a metal selected from the group comprised of: gold, chromium palladium titanium germanium, or combinations thereof, for example chromium/gold, palladium/germanium, titanium/gold alloys, etc.
Advantageously, the lattice filling coefficient, i.e. the ratio between the surface of the cuts and the surface of uncut zones, is set between 40% and 60%, for example it is set at 50%.
Advantageously, said number of cuts is a prime number that is selected from the group comprised of: five, seven, eleven, thirteen, seventeen, nineteen, twenty-three, twenty-nine, thirty-one, thirty-seven, forty-one, forty-three, and forty-seven.
In particular, said number of cuts is an odd number that is a multiple of a prime number, said prime number greater than or equal to five, said odd number selected from the group comprised of: fifteen, twenty-one, twenty-five, twenty-seven, thirty-three, thirty-five, thirty-nine, forty-five, forty-nine, fifty-one.
According to another aspect of the invention a laser device comprises a laser resonator as above defined.
Advantageously, said laser device comprises on at least one plane an ordered group of such laser resonators.
In a further exemplary embodiment, the perimeter of the resonator is linear, and the cuts which are formed by adjacent slits are at a predetermined distance from each other—This way, the efficiency of the vertical emission from the grating mode radiating vertically, its optical losses, as well as the amount of feedback can be controlled. Moreover, the output power in the vertical direction can be maximised, at the same time ensuring that only the symmetric mode can reach the laser threshold, producing regular, single-lobe beam profiles.
Advantageously, for THz QC lasers with a slit separation that is 0.6-0.7 times the grating period can be provided, to obtain the best lasing conditions.
According to a further aspect of the invention, a method is provided for making a planar resonator which is adapted to be associated to a laser, said method comprising the steps of:
wherein said first wave-guiding layer is formed on said active region according to a lattice of cuts,
and wherein said cuts are made by at least two adjacent slits and a central zone in which metal is present.
Advantageously, the perimeter can be circular, and the cuts are radial cuts, said radial cuts can be made in such a way that the number of said cuts is a prime number or an odd number that is a multiple of a prime number, said prime number greater than or equal to five.
Alternatively, the resonator is linear, and the cuts which are formed by adjacent slits are at a predetermined distance from each other so as to maximize the output power in the vertical direction, and produce regular, single-lobe beam profiles. In particular, for THz QC lasers best results are generally achieved with a slit separation that is 0.6-0.7 times the grating period.
The invention will be made clearer with the following description of some exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings wherein:
In the following description, as circular resonator a disc or ring plane resonator is intended, as well as an elliptical resonator, or a rectangular circularly or elliptically ending resonator is used, which can operate according to the mode “whispering gallery”.
With reference to
In
According to an aspect of the invention, the number of cuts 4 can be a prime number, for example five, seven, eleven, thirteen, seventeen, nineteen, twenty-three, twenty-nine, thirty-one, thirty-seven, forty-one, forty-three, forty-seven etc., or it is an odd number that is a multiple of a prime number, such prime number greater than or equal to five, for example fifteen, twenty-one, twenty-five, twenty-seven, thirty-three, thirty-five, thirty-nine, forty-five, forty-nine, fifty-one. In the specific case of
As shown in
In particular, cuts 4 form a circumferential lattice which has a lattice filling factor, i.e. the ratio between the surface of cuts 4 and the surface of uncut zones 5, preferably set between 40% and 60%, or vice-versa. In the exemplary embodiment of
The disc or ring 1 overall thickness may be even only one micron, even if, as in case of double metal quantum-cascade laser devices, i.e. laser devices in which an active region 3 that is arranged between two metal layers 2 and 6, the overall thickness is set, for example, between five and fifteen microns.
In
In
In
In
The pumping step of the laser resonator can be carried out by optical, electrical excitation, or by another excitation mode.
In
This is an advantageous solution for a disc laser resonator, while it is less useful for a ring laser resonator, like that of
The outer metal coating may be about 10-300 nm thick, and may be made of titanium/gold alloy, and may be joined to the wafer by a thermocompression procedure. The radial cuts may have various lengths, for example the length may range from 160 to 210 μm, and may be made of a thermally evaporated Cr/Au metallization that is laid by means of an optical lithography and a lift-off procedure. The cuts in the doped semiconductor layer are made by Induced Coupled Plasma Reactive Ion Etcher (ICP-RIE). In particular, the upper contact is engraved by using the metal as a self-aligned mask, whereas the central portions are engraved by a photoresist mask. Successively, the obtained devices are indium-welded on a copper base, and then they are joined with the lead wire and mounted on the cold finger of a liquid helium cryostat.
The diameter, the number of cuts and the length of the radial direction of the cuts, depends upon, or influence, the wavelength of the radiation that is emitted by the laser inside the semiconductor material. In possible embodiments, the disc has a diameter set between 170-180 micron, and the length of the cuts is about 20 micron. In this case, the wavelength of the emitted radiations is about 100 micron.
In alternative, instead, if the number of cuts is changed, for example, if it is lowered from seventeen to thirteen, or if it is increased to nineteen, and so on, the number of oscillation periods of the laser radiations respectively decreases or increases to thirteen or to twenty-three, and causes a variation of the wavelength of the laser radiations.
By choosing a prime number of cuts 4, the resonator is forced to “laser” only within the lattice that is formed by cuts 4 and by portions 5, provided the active material is excited. In the case of
For better understanding this aspect, it is intuitively observed that, in a disc resonator, the permitted optical modes, which travel along the circumference, are characterized by the condition that the component of the wave vector of the radiations in the azimuth direction ka satisfies the relationship:
k
a=2πn/L
where L is the length of the circumference of the disc and n an integer number. This condition simply derives from by the need that the electromagnetic wave, after travelling along the whole disc circumference, has exactly the same phase value as before.
The lattice, in order to effectively lead the radiations in a vertical direction, must work on the second order of diffraction, in other words the following relationship must be fulfilled:
k
a=2π/Λ
where Λ is the pitch of the lattice, i.e. L=nΛ. In a true whispering gallery mode of a laser disc,
k
a=2π/λ
where λ is the radiations wavelength in the semiconductor material, therefore the disc size and the lattice pitch should be selected such that
Λ=λ.
However, in a disc laser a component of the wave vector can exist also in a radial direction kr; consequently, for a prefixed λ value, a laser can work according to a plurality of modes, in which:
k
a≧2π/λ,
provided, in each case, that
2π/λ=√(ka2+kr2).
Therefore, the laser might at any rate work according to a mode in which, for example:
k
a=π/Λ
(first order lattice) that however would have:
L=2nΛ,
or still according to a mode in which
L=3nΛ
(where n is still an integer number), and so on, provided
kr>0.
This is exemplified in
To assure that only the equation
k
a=2π/Λ
is satisfied, the laser disc must be designed such that
L=mΛ,
where m is a prime number.
On the same subject, see
Let still be λ the wavelength of the radiations emitted by the laser, Λ the lattice pitch, i.e., the distance between two cuts, and K=2π/λ the wave vector of the radiations: the lattice behaves as a second order lattice if the component of K in the circumferential or azimuth direction of the lattice is 2π/Λ (
Actually, if the number of cuts is a prime number, or if it is an odd number that is a multiple of prime numbers that are higher than five, along the boundary of the laser resonator, the lattice prevents the circumferential component from being π/Λ, therefore the lattice still behaves as a second order lattice and ensures a vertical emission.
This theoretical imposition is used for frequencies that are in the range of terahertz, even if the number of cuts is not a prime number, but it is an odd number that is a multiple of a prime number, said prime number greater than or equal to five. In this case, in fact, since the resonator is circular, even if it is theoretically possible that the resonator “lasers” on zones that are different from the lattice, i.e. towards the centre of the disc, at a distance that is a multiple of the wavelength, such areas would be impossible to be attained, due to the small diameter of the disc.
If the lattice coupling coefficient is set high enough (˜1/L or higher), the laser can work only according to a true whispering gallery mode, therefore the lattice is a true second order lattice and extracts most of the emitted radiations vertically.
The modes with kr≠0 are modes that extend further towards the disc centre; in a device laser they might be preferred with respect to the true whispering gallery mode, which has less dispersion (due to the introduction of the lattice) and a higher electric pumping effectiveness (see the figure).
If the coupling coefficient is too low, such modes can be eliminated also by reducing the electric pumping in the central zone. This can be obtained with the exemplary embodiment of
Since the upper metallization acts also as an electric contact, it is possible to replace each cut 4 with two cuts that are close to each other, 4a and 4b, or with a plurality of slightly spaced apart cuts; this improves the electric pumping without affecting the vertical emission efficiency, and without changing the resonance mode. In fact, portions 5 remain unchanged, whereas the two (or more) cuts 4a and 4b are spaced apart by portions 5a, thus indeed improving the electric pumping without affecting the overall performances. This is particularly advantageous by laser devices that operate with higher wavelength, in which the width of a single cut would be too large to allow a uniform injection of electrons.
the power that is achieved by the dotted curve 30, which refers to vertically emitted power responsive to the current intensity in the case of a resonator that has sixteen cuts and a 170 micron diameter,
the power that is achieved by continuous curve 31, which shows the relationship of emitted power versus current intensity in the case of a resonator with seventeen cuts and a 182 micron diameter. The diagram is limited to the limit at which the current can be tolerated. Evidence is given that the achieved power is extremely higher, and that the derivative of the power is even much higher, versus current intensity. This shows the large power that can be extracted with such a laser geometry, which forces the laser to emit only along the lattice.
With reference to
With reference to
With reference to
This feature improves the electric pumping without affecting the vertical emission efficiency, and without changing the resonance mode. In fact, the two cuts 14a and 14b are spaced apart by portions 15a, and the electric pumping is improved without affecting the overall performances. This is particularly advantageous by laser devices that operate with higher wavelength, such as THz QCLs, in which the width of a single cut (see cut 14 in
The grating period is substantially D1+D2, since the width of the slits 14a and 14b is negligible (about 1/10 of the grating period). In
As shown in
Distance D1 is chosen equal to a certain fraction of the grating period. Such distance D1 is chosen so as to provide the necessary amount of surface losses from the device to ensure that laser action can be achieved on a vertically emitting optical mode and with a maximum efficiency. The value of such distance depends on the characteristics of the laser waveguide and gain material.
As shown in
More precisely,
Alternatively, in a linear resonator 11″″, as shown in
The foregoing description of a specific embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such an embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.
| Number | Date | Country | Kind |
|---|---|---|---|
| PI2008A000038 | May 2008 | IT | national |
This application is a continuation-in-part of PCT/IB2009/005413 filed Apr. 28, 2009, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/IB2009/005413 | Apr 2009 | US |
| Child | 12940209 | US |
